Systems have been developed to activate and detect remote activatable markers positioned, for example, in a selected item or object. The markers generate a signal used to detect the presence of the marker. Many of the activatable markers are hard-wired to a power source or other equipment external from the object. Other systems have been developed that utilize resonating leadless markers, also referred to as wireless active markers, positionable at or near a selected target. These wireless active markers are typically activated by a remote excitation source that generates a strong continuous excitation signal. The activated markers generate a detectable marker signal that must be distinguished from the strong continuous excitation signal and then analyzed to try to accurately determine the target's location. The process of distinguishing a weak marker signal from the strong continuous excitation signal, while maintaining sufficient accuracy and repeatability for determining the marker's location, has proven to be very difficult.
In the case of a verification device for medical tube placement, U.S. Pat. No. 5,325,873 to Hirschi et al. teaches a system that detects the general position of an object within a body of tissue. The detection system includes a three-axis resonant-circuit target attached to the object. A separate remote hand-held detection probe has a pair of parallel and coaxially aligned transmitter/sensing coils. The transmitter sensing coils generate a current that determines whether a return signal strength of the target is great enough to be counted as a valid signal. The hand-held detection probe also has a pair of receiver coils positioned within each of the transmitter coils and connected in a series-opposed fashion. The hand-held detection probe also has a visual display coupled to the receiver coils and configured to indicate the direction in which the probe should be moved to center the detection probe over the selected object. While the system of Hirschi et al. is usable to detect the presence of an object in a body, the system is not usable for tracking and monitoring an object in real time during radiation therapy treatment.
Recent advances in radiation therapy are providing new avenues of effective treatment for localized cancer after the cancer's position has been determined. The treatments include 3D conformal external beam radiation, inverse modulated radiation therapy (IMRT), stereotactic radiosurgery and brachytherapy. These newer treatment modalities deliver greater doses of radiation to a tumor, which accounts for their increased effectiveness when compared to traditional standard external beam irradiation.
A dose response relationship for radiotherapy exists for most cancers, so dose escalation is often necessary to achieve continued improvements in the management of localized cancers with radiotherapy. As the radiation dose is increased, the volume of adjacent normal tissue irradiated around the cancerous target can be decreased by maintaining a tighter treatment margin around the target. The size of the treatment margin, however, must be sufficient to accommodate potential tumor motion before or during radiation therapy. As an example, movement of a tumor in the prostate often occurs during radiation treatment primarily due to patient breathing, rectal and bladder filling and emptying, which consequently move the prostate. Accordingly, it is highly desirable to monitor actual tumor motion in real time during the delivery of radiation therapy to minimize treatment margins while ensuring that the tumor does not move out of the treatment volume.
It is known that the introduction of solid materials in the path of a high energy photon or radiation beam during radiation therapy displaces electrons from the solid materials. To a lesser extent, such interaction also generates secondary photons of lower energy than the primary photons of the radiation beam. The displaced electrons and secondary photos are scatter products that contaminate the beam. Because the scatter products have a lower energy than the primary photons, the scatter products more readily damage superficial tissues of the body, such as the dermis and the subcutaneous layer, than do the primary photons. The primary photons in the beam penetrate the patient to irradiate the target, but damage to the superficial tissues by scatter products may limit the total dose that can be delivered to the patient.
It is also known that the high energy radiation therapy photon beam is attenuated as it passes through solid materials in its path. Managing radiation treatments includes defining the geometry of a plurality of radiation fields to be used in the treatments and specifying the radiation dose to be delivered with each of the fields. This stage of treatment management is referred to as “treatment planning,” and the control and measurement of dose distribution is termed “dosimetry.” Attenuation of the therapy beam by solid materials, such as beam filters and other accessories, is typically included in the computations of dose distribution in the target tissue in the treatment planning process.
A further implication of the attenuation caused by components dwelling in the radiation beam is the appearance of artifacts in x-ray images collected for the purpose of verifying patient positioning. For example, structural details of the components in the path of the radiation used for imaging will appear in the images.
In light of the problems of beam contamination and attenuation, it is not desirable to place structures in the path of the radiation beam unless the benefits of doing so outweigh the resulting contamination and attenuation. For example, it is often necessary to position patient tabletops, blocking beam trays, and immobilization devices in the radiation beam during radiation therapy. Accordingly, even though it may be desirable to position additional equipment in the radiation beam, this is difficult because of the additional beam contamination and/or attenuation.